Flash TTL doesn't work properly with B&W?

The T90 was the first Canon camera to support through-the-lens (TTL) flash metering. This measured the actual light levels reaching the film by measuring reflected light off the film (OTF), shutting down the flash unit once the film is sufficiently exposed. The measurement is calculated using the average reflectivity of color negative film. This system was also used on Canon's new EOS system, making the T90 the only non-EOS Canon body compatible with TTL Canon flashes[6].

Hmm. I never really new how the flash ttl worked, I guess the different reflectivity of B&W film changes how the film is exposed with black and white? I haven't had alot of experience with flash use, but I plan on it.

Be careful you do not confuse Off-The-Film (OTF) with Through-The-Lens. OTF will be affected by film reflectivity; TTL will not.
Hasselblad's ProFlash was designed to operate with their 503Cx (OTF) system. and in the manual for its operation, they list compensations for the reflectivity of various films. Unfortunately, many of the films listed are no longer available, and many new films are not listed, so one must establish their own particular "tweak" factor.

I've used both TTL and OTF (Olympus OM is OTF) and I would choose OTF, without question - worth the extra effort.